1. Field of the Invention
The present invention is directed to a method for producing an image of an examination subject by computed tomography wherein an X-ray source that moves around the examination subject for the production of the image.
2. Description of the Prior Art
In a computed tomography, projections are registered from the examination subject while an x-ray source moves around the examination subject, Data allocated to these projections are employed for the production of the image If the examination subject or parts of the examination subject move while the projections are being registered, the image can exhibit motion artifacts that can result in an unsharp image. This problem occurs particularly in images of the heart or of heart-proximate lung structures when the projections were registered during the rapid contraction phase of the heart.
In order to at least alleviate motion artifacts when registering the heart or heart-proximate lung structures, German OS 198 54 939 discloses a method for generating CT images of a body region having quiescent and motion phases and moving periodically by means of a CT apparatus having an x-ray source moved around the body of the life form to be examined. In this method, projections for image reconstruction employ only data allocated to the projections that were acquired during a quiescent phase.
A disadvantage of this method, however, is that it is limited to a motion of a body region that moves periodically with quiescent or motion phases.
Morneburg (editor), xe2x80x9cBildgebende Systeme fxc3xcr die medizinische Diagnostikxe2x80x9d, Publicis MCD Verlag, Erlangen, 1995, pages 136 and 137, describes a method for reducing motion artifacts that is also suitable for non-periodic movements of the examination subject or parts of the examination subject The movements, for example, are generated by peristaltics, respiration, tremor or general unrest on the part of the examination subject or parts of the examination subject.
In this method, referred to as the multi-scan technique, the x-ray source moves repeatedly around the examination subject and the data allocated to the projections are subsequently averaged Motion artifacts that occur are reduced as a result. A disadvantage of this method is the increased radiation dose that the examination subject is exposed to due to the multiple movement of the x-ray source around the examination subject. Further, data that are allocated to a movement of the examination subject are also employed for the production of the image.
An object of the invention is to provide a method that creates pre-requisites for alleviating negative influences of a not necessarily periodic movement of the examination subject or parts of the examination subject during the registration of the projections
The object is achieved in a method for producing an image of an examination subject by computed tomography wherein an x-ray source moves around the examination subject for producing the image, wherein a number of projections serving for the production of the image are registered during at least one revolution of the x-ray source around the examination subject, comprising the following method steps:
a) determining those data of the projections that are falsified by a movement of the examination subject; and
b) replacing at least the data of a projection falsified by the movement of the examination subject by the data complementary thereto.
In the inventive method, thus, projections are registered from the examination subject during at least one revolution of the x-ray source around the examination subject. Subsequently, the data allocated to the projections that are falsified by the movement of the examination subject are identified and are replaced by their complementary data. The data and their complementary data can, for example, be acquired from projections in parallel geometry, with the projections that are allocated to the data or their complementary data exhibiting a projection angle offset by 180xc2x0. Since modern computed tomography systems usually register fan projections, parallel projections are generated therefrom only by suitable interpolation rules and resorting rules, for example by the known technique of re-binning. A parallel projection is then composed of measured values of fan projections measured at different points in time. For example, the measuring point in time of a central channel can then be defined as measuring point in time of a parallel projection.
An advantage of inventive method is that only data that are not falsified by a movement of the examination subject or parts of the examination subject are employed for producing the image. It is thus not necessary that the movement of the examination subject ensue periodically.
In order to identify the falsified data, a deviation of the data of a projection from its complementary data is determined according to one version of the invention, wherein data or their complementary data are considered falsified when the deviation exceeds an upper limit value "sgr"s. The background of this consideration is that data of a projection and their complementary data are identical given an immobile examination subject. A deviation of the data of a projection from their complementary data, thus, is an indicator for the movement of the examination subject.
In a preferred version of the invention, parallel coordinates are allocated to the projections. The deviation of the data of the projection from their complementary data is determined according to the following method:
a) calculating difference signals xcex94S1(xcex8, p) from the data of the projection and their complementary data for each projection angle xcex8 between 0 and xcfx80 of the projection and each channel pxcex5[v, P] according to the following equation:
xcex94S1(xcex8, p)=S(xcex8, p)xe2x88x92S(xcex8+xcfx80,xe2x88x92p)xe2x80x83xe2x80x83(1) 
whereby the signals S(xcex8, p) are allocated to the projections for projection angles xcex8 between 0 and xcfx80 and the signals S(xcex8+xcfx80, xe2x88x92p) are allocated to their complementary data; and
b) calculating the deviation as standard deviation "sgr"1(xcex8) of the difference signals xcex94S1 (xcex8, p) for each projection angle xcex8 between 0 and xcfx80 via the channels pxcex5[PS, P74] according to the following equation:
"sgr"1(xcex8)="sgr"p{xcex94S1(xcex8,p)},xe2x80x83xe2x80x83(2) 
whereby [Ps, Pe]⊂[xe2x88x92P, P].
Inventively, thus, difference signals xcex94S1(xcex8, p) are formed for each projection angle xcex8 between 0 and xcfx80 and for each channel pxcex5[xe2x88x92P, P]. Subsequently, the standard deviation "sgr"1(xcex8) of the difference signals xcex94S1(xcex8, p) is formed. The standard deviation "sgr"1(xcex8) can be inventively formed over all channels pxcex5[xe2x88x92P, P]. When data of a projection are falsified due to a movement on the part of the examination subject, thus, all data of this projection are replaced by their complementary data. The standard deviation a(e), however, also can be formed over sub-regions [Ps, Pxcex8] of the channels p. Then, in particular, locally limited motion artifacts, i.e. movements on the part of sub-regions of the examination subject, can be discovered and the data falsified as a result can be limited in the projection. As a result, it is possible to replace only the data of the projection falsified due to the partial movement of the examination subject by their complementary data.
Whether the data of the projection or their complementary data are falsified by the movement of the examination subject, however, cannot be recognized on the basis of the determination of the deviation of the data of a projection from their complementary data. Another allocation is thus necessary as to whether the falsified data are allocated to a projection angle xcex8 between 0 and xcfx80 or to a projection angle xcex8 between xcfx80 and 2xcfx80. According to one embodiment of the invention, the deviations of the data of a projection from data of the following projection are therefore determined for all projection angles xcex8 that are allocated to at least one revolution of the x-ray source. Further, a projection angle xcex8max is determined that corresponds to the maximum deviation of the deviations. The falsified data then have projection angles xcex8 between 0 and xcfx80 allocated to them when xcex8max lies between 0 and xcfx80 and the falsified data are allocated to projection angles xcex8 between xcfx80 and 2xcfx80when xcex8max lies between xcfx80 and 2xcfx80.
According to a preferred version of the invention, parallel coordinates are allocated to the projections. The deviations of the data of a projection from data of the following projection are determined according to the following method:
a) calculating difference signals xcex94S2(xcex8, p) from data of a projection and data of its following projection for each projection angle xcex8 between xcex94xcex8 and 2xcfx80 and each channel pxcex5[xe2x88x92P, P], whereby the projection angles xcex8of  the projection and its following projection differ by xcex94xcex8, according to the following equation:
xcex94S2(xcex8,p)=S(xcex8, p)xe2x88x92S(xcex8xe2x88x92xcex94xcex8, p)and xe2x80x83xe2x80x83(3) 
b) calculating the deviation as standard deviation "sgr"2(xcex8) of the difference signals xcex94S2(xcex8, p) for each projection angle xcex8 between xcex94xcex8 and 2xcfx80over the channels pxcex5[xe2x88x92P, P] according to the following equation:
"sgr"2(xcex8)="sgr"p{xcex94S2(xcex8,p)}xe2x80x83xe2x80x83(4) 